1. Field of the Invention
The present invention relates to charge transfer devices and, more particularly, to a charge transfer device designed to transfer and convert signal charges into corresponding output voltages, for example, a linear sensor, an area sensor, or a delay element.
2. Description of the Related Art
FIGS. 1A and 1B show a conventional CCD-type solid-state image sensing device, in which 1A is a plan view, and 1B shows the waveform of an output voltage V.sub.out.
Referring to the figures, a plurality of photoelectric conversion elements (hereinafter referred to as "photo-sensors") 1 form pixels which are arranged in a matrix form. Vertical registers 2 are formed to respectively correspond to vertical lines (columns) of photosensors 1. A horizontal register 3 horizontally transfers signal charges transferred by the vertical registers 2. An output buffer 4 converts each signal charge outputted from the horizontal register 3 into a voltage V.sub.out.
In the conventional solid-state image sensing device or other charge transfer device, the relationship between the intensity of light incident on a photosensor 1 or the amount of signal charge generated by the incident light and the corresponding output voltage of the output buffer 4, that is, light intensity (amount of charge)-output voltage characteristic, depends largely on the supply voltage and temperature.
FIGS. 2A and 2B show the charge-output voltage characteristic in a standard state 2A and also in a state where a condition (temperature) has changed 2B. The output voltage has a tendency to rise as the temperature becomes higher. The tendency is particularly noticeable when the light intensity is low. However, even when the temperature is low, the output voltage is relatively high in a region where the light intensity is high. Thus, the slope of the characteristic curve delicately changes with temperature. Further, a change in the supply voltage causes the charge-output voltage characteristic to change, as a matter of course.
The fact that the charge-output voltage characteristic depends largely on the supply voltage and temperature means that for the same light intensity (amount of charge), the output voltage varies to a considerable extent in accordance with a change in the supply voltage or temperature. Accordingly, correction must be made when the characteristic has changed. Such correction can readily be made by the existing technique, provided that a quantity to be corrected can be judged.
Incidentally, there has heretofore been no method of appropriately detecting a quantity of change in the charge-output voltage characteristic. The conventional practice is to make a comparison between a subject signal charge serving as a reference white level and a reference black level (i.e., the reference black level of the output signal, optical black, or a subject signal serving as a reference black level). The conventional method suffers, however, from the problem that no characteristic can be detected unless a subject signal charge serving as a reference white level is inputted either at all times or every time a state change occurs. Further, it is difficult in practice to take light from the subject serving as a reference white level in one field. The reason for this is that the brightness of the subject cannot conveniently reach the reference white level when imaging process is actually initiated.